(i) Field of the Invention
The present invention relates to an optical resolution method of functional compounds and a racemization method of optically active bifunctional compounds.
(ii) Description of Related Art
Bifunctional compounds such as diols, dihydroxybiaryls and hydroxyoximes are neutral or slightly acidic, and therefore they do not usually interact on a resolving reagent, when they are intactly present without any additional treatment. Furthermore, each of these compounds has plural functional groups, and for this reason, it is extremely difficult to resolve them. Conventional optical resolution methods, of which there are only a small number, comprise converting the bifunctional compound into a derivative such as an ester, and then resolving it, and resolving reagents which are used in these methods are not easily available. In consequence, these conventional methods are not practical.
For example, as a purely chemical optical resolution method, there is known a process which comprises resolving, with cinchonine or cinchonidine, a phosphate obtained by reacting racemic 2,2'-dihydroxy-1,1'-binaphthyl with phosphorus oxychloride [Org. Synth., 67, 1 (1988)], or resolving the same with optically active 2-aminobutanol (Synthesis, 222 (1990), and then reducing with lithium aluminum hydride [Org. Synth., 67, 13 (1988)] to obtain optically active 2,2'-dihydroxy-1,1'-binaphthyl. However, this process requires the complicated operation of converting 2,2'-dihydroxy-1,1'-binaphthyl into the phosphate, optically resolving the same, and then removing a phosphoric acid portion therefrom by the reduction, and in the last step of this process, phosphine which is a poisonous gas is produced, which is inconvenient.
Moreover, in the conventional optical resolution methods, the necessary enantiomer of the optically active compound is used and removed from a pair of enantiomers, and the remaining unnecessary enantiomer is useless and uneconomical. Accordingly, it is important how the unnecessary enantiomer is effectively racemized and then returned to the necessary enantiomer. However, the racemization of optically active binaphthols has scarcely been researched, and only methods of reacting with hydrochloric acid in dioxane and other methods of reacting with potassium hydroxide in butanol are known.
In these methods, however, the racemization is slow and volumetric efficiency is also low. Therefore, there are desired a racemization method of optically active binaphthols which permits the racemization to proceed rapidly and which is excellent in volumetric efficiency and is economical, a racemization method which does not require any especial racemization step, and an optical resolution method which permits obtaining a necessary enantiomer.